1. Field of the Invention
The present invention relates generally to a calibration apparatus and method for controlling the phase and amplitude of a signal in a smart antenna multicarrier communication system, and in particular, to an apparatus and method for transmitting a calibration signal on the remaining carriers after allocating data to carriers, thereby increasing the efficiency of frequency resource utilization for the data signal.
2. Description of the Related Art
A smart antenna system is a communication system that uses a plurality of antennas to automatically optimize a radiation pattern and/or a reception pattern according to a signal environment. From the perspective of data signal transmission, the smart antenna system transmits a signal with a desired strength in an intended direction at a minimum power level by beamforming. The use of the smart antenna enables a Base Station (BS) to direct a signal only to a desired Mobile Station (MS) through beamforming. Therefore, compared to omnidirectional signal transmission to all MSs, the smart antenna reduces power required for signal transmission and interference, as well. Since the smart antenna applies directionality to a transmission/received signal by actively locating an intended MS, interference to other MSs within the same cell can be minimized. Thus, the BS can allocate the remaining available power to other MSs and the reduced interference with other cells leads to the increase of BS channel capacity.
A wireless internet service system based on Orthogonal Frequency Division Multiple Access (OFDMA) uses a wide frequency bandwidth and transmits a signal from a BS to one MS at a higher power level than in a conventional system. Thus, a cell radius is small. Application of the smart antenna to the wireless internet system advantageously increases BS channel capacity.
In application of the smart antenna system to a multicarrier OFDMA system, beamforming is performed by using a beamforming weight vector for each orthogonal frequency carrier of each antenna such that each antenna beam is steered in a chosen direction. The beams must reach the antennas without any change prior to transmission over the air, but they experience distortions in their phase and amplitude due to non-linear components in the BS. Thus, calibration is needed to control the phase and amplitude of the signals. The total performance of the smart antenna technology depends on the accuracy of the calibration, that is, the accuracy of beam directionality and minimization of phase mismatch. The calibration is commonly applied to a downlink directed from a BS to an MS and an uplink directed from an MS to a BS.
FIG. 1 is a block diagram of a conventional calibration apparatus in a smart antenna system. Referring to FIG. 1, a transmission (Tx) calibration signal is transferred in the following manner. First, a calibration signal generated from a calibration processor and controller 110 under the control of other layers of the BS 109 is provided to a baseband module 108. The calibration signal is then transmitted to antennas 101 through a Radio Frequency (RF) module. The RF module oversamples the calibration signal in a Digital UpConverter (DUC) 106, modulates the oversampled signal to an RF signal in a Tx module 104, and transmits the modulated signal to the antennas 101 through a Transceiver Control Board (TCB) 103 and a coupler-splitter 102. Meanwhile, the calibration signal is coupled in the coupler-splitter 102 and transferred in a calibration path. Specifically, this calibration signal returns to the calibration processor and controller 110 through a TCB 103, a reception (Rx) module 105, and a Digital DownConverter (DDC) 107 in a Tx calibration path.
As to an Rx calibration signal, a calibration signal generated from the calibration processor and controller 110 passes through a DUC 106, a Tx module 104, and a TCB 103 in an Rx path and is coupled to signals received at the antennas 101 in a coupler-combiner 102. The coupled signal returns to the calibration processor and controller 110 through a TCB 103, an Rx module 105, a DDC 107, and the baseband module 109 in an Rx calibration path.
As described above, calibration vectors are estimated for Tx calibration and Rx calibration by computing differences in phase and amplitude between calibration signals generated from the calibration processor and controller 110 and the calibration signals fed back from the Tx and Rx paths.
FIG. 2 illustrates the principle of calibration in the conventional smart antenna system. A Tx or Rx calibration signal C(t) experiences variations in its phase and amplitude as it travels in a path running to antennas and in a feedback path. Given N antennas, the calibration signal C(t) is received from N paths. Thus, according to Equation 1:
                                                        C              1                        ⁡                          (              t              )                                =                                    α              1                        ⁢                          C              ⁡                              (                t                )                                      ⁢                          e                              j                ⁢                                                                  ⁢                                  θ                                      1                    ,                    cal                                                                        ⁢                          e                              j                ⁢                                                                  ⁢                                  θ                  feedback                                                                    ⁢                                  ⁢                                            C              2                        ⁡                          (              t              )                                =                                    α              2                        ⁢                          C              ⁡                              (                t                )                                      ⁢                          e                              j                ⁢                                                                  ⁢                                  θ                                      2                    ,                    cal                                                                        ⁢                          e                              j                ⁢                                                                  ⁢                                  θ                  feedback                                                                    ⁢                                  ⁢        ⋮        ⁢                                  ⁢                                            C              N                        ⁡                          (              t              )                                =                                    α              N                        ⁢                          C              ⁡                              (                t                )                                      ⁢                          e                              j                ⁢                                                                  ⁢                                  θ                                      N                    ,                    cal                                                                        ⁢                          e                              j                ⁢                                                                  ⁢                                  θ                  feedback                                                                                        (        1        )            where Cn(t) denotes a feedback calibration signal from an nth path and αn denotes attenuation in the nth path. θN.cal is a phase factor for nth path and θfeedback is a phase factor for feedback path.
For calculation of a calibration vector, a coupler characteristic Rcoupler from each path must be eliminated and for beamforming, the relative phases of the N antennas must be matched. Calibration vectors are computed by Equation 2.
                                          w                          c              ,              1                                =                      conj            ⁡                          [                                                                                          C                      1                                        ⁡                                          (                      t                      )                                                        /                                      R                                          coupler                      ⁢                                                                                          ⁢                      1                                                                                        C                  ⁡                                      (                    t                    )                                                              ]                                      ⁢                                  ⁢                              w                          c              ,              2                                =                      conj            ⁡                          [                                                                                          C                      2                                        ⁡                                          (                      t                      )                                                        /                                      R                                          coupler                      ⁢                                                                                          ⁢                      2                                                                                        C                  ⁡                                      (                    t                    )                                                              ]                                      ⁢                                  ⁢                                  ⁢        ⋮        ⁢                                  ⁢                              w                          c              ,              N                                =                      conj            ⁡                          [                                                                                          C                      N                                        ⁡                                          (                      t                      )                                                        /                                      R                                          coupler                      ⁢                                                                                          ⁢                      N                                                                                        C                  ⁡                                      (                    t                    )                                                              ]                                                          (        2        )            
Assuming that beamforming weight vectors for antennas are Wb1, Wb2, Wbn, beamforming weight vectors calculated taking antenna paths into account are Wb1Wc1, Wb2Wc2, WbnWcn.
The calibration must be performed periodically for all carriers to use the smart antenna in a multicarrier communication system such as OFDMA. This calibration requires allocation of frequency resources to a calibration signal. However, the additional frequency resource allocation for the calibration signal leads to dissipation of frequency resources and thus there is a need for a technique of solving this problem.